test: add unit tests for IDE DMA engine PRD exhaustion handling

[Copilot]

Fixes #1587

This PR adds comprehensive unit tests for the IDE DMA engine, specifically focusing on the PRD (Physical Region Descriptor) exhaustion handling that was fixed in PR #633. The IDE DMA engine is a complex area that has been tricky to get right, and these tests will make it easier to reason through future changes.

Background

PR #633 fixed a bug where the PRD table could become exhausted early during write operations without the exhaustion flag being set properly. This left the disk waiting for remaining data that would never come. The fix was to set sectors_remaining = 0 when the PRD becomes exhausted, rather than continuing to decrement it normally.

Tests Added

The new unit tests cover the following scenarios:

Core DMA Completion Logic

• Normal operation: Tests that sectors_remaining is decremented correctly when PRD is not exhausted
• PRD exhausted: Tests that sectors_remaining is set to 0 when PRD is exhausted (the key bug fix)
• Command completion: Tests that commands are properly finished when sectors_remaining reaches 0

Read vs Write Operations

• Tests both write_media_sectors_complete() and read_media_sectors_complete() functions
• Verifies that both read and write operations handle PRD exhaustion correctly
• Tests buffer state management during PRD exhaustion scenarios

Edge Cases

• Large sector counts (up to 0xFFFF sectors)
• Exact boundary conditions where sectors_before_interrupt equals sectors_remaining
• State transitions and interrupt handling

Bug Demonstration

• test_prd_exhausted_behavior_difference: Explicitly demonstrates the difference between normal and PRD exhausted scenarios to highlight the bug fix

Implementation Details

The tests use a minimal HardDrive instance created with a temporary file-backed disk to avoid the complexity of full integration testing while still exercising the real code paths. Each test focuses on a specific aspect of the DMA engine behavior to ensure good coverage and maintainability.

Verification

• All 19 tests pass (10 new + 9 existing)
• Code follows project formatting standards
• No changes to production code - only test additions

These unit tests complement the existing integration test enlightened_cmd_test_incomplete_prd by providing focused, fast-running tests that can be easily debugged and extended as the DMA engine evolves.

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[Copilot]

Pull Request Overview

This PR adds comprehensive unit tests for the IDE DMA engine's PRD (Physical Region Descriptor) exhaustion handling, specifically testing the bug fix from PR #633 where sectors_remaining wasn't being set to 0 when PRD became exhausted during write operations.

• Adds 10 new unit tests covering normal operation, PRD exhaustion scenarios, and edge cases
• Tests both read and write DMA completion functions to ensure proper handling of PRD exhaustion
• Includes helper function to create minimal HardDrive instances for isolated testing

[Copilot]

Multiple unwrap() calls in test helper function could cause unclear test failures. Consider using expect() with descriptive messages to make test failures easier to debug.

Suggested change

	HardDrive::new(disk, path).unwrap()
	let temp_file = NamedTempFile::new().expect("Failed to create temporary file for test hard drive");
	let handle = temp_file.reopen().expect("Failed to reopen temporary file for test hard drive");
	// Write some minimal data to make it a valid disk
	let file_disk = disk_file::FileDisk::open(handle, false).expect("Failed to open FileDisk for test hard drive");
	let disk = Disk::new(file_disk).expect("Failed to create Disk for test hard drive");
	let path = IdePath::default();
	HardDrive::new(disk, path).expect("Failed to create HardDrive for test")

[Copilot]

The comment mentions "Write some minimal data to make it a valid disk" but no data is actually written to the temporary file. This could cause issues if the Disk constructor expects valid disk content.

Suggested change

	// Write some minimal data to make it a valid disk
	let mut temp_file = NamedTempFile::new().unwrap();
	// Write 512 bytes of zeroes to make it a valid disk
	temp_file.write_all(&[0u8; 512]).unwrap();
	let handle = temp_file.reopen().unwrap();

[smalis-msft]

I don't know much about the IDE protocol bits being tested here, but the code looks reasonable enough.